docs: def get_entity_by_identifier(self, url: str, identifier: str) -> Optional[NamespaceEntry]:
          """Get a given entity by its url/identifier combination."""
          entry_filter = and_(Namespace.url == url, NamespaceEntry.identifier == identifier)
          return self.session.query(NamespaceEntry).join(Namespace).filter(entry_filter).one_or_none()